Exceptional situations, in particular those requiring a safe descent in proximity to the relief, require fast decisions to be taken under conditions of extreme stress.
The first few minutes after detecting an incident are critical and a bad decision can have serious consequences on the continuation of the flight or the workload imposed on the crew to deal with it or rectify it.
The problems posed by an emergency descent have been mentioned for example in the following publications:                the safety bulletin of the FAA, according to which the emergency descent involves concepts of steep angles of descent, high speeds and sweaty palms,        the article “Training and proficiency” (AMCC III.d), according to which a complex emergency situation would involve implementing one or more emergency procedures, redefining a trajectory and a descent profile and communicating to the air traffic control the three essential items of information in regard to air traffic control, namely the nature of the emergency situation, the quantity of fuel remaining and the number of persons aboard. Furthermore, in an emergency situation, the paramount obligation of the pilot is to continue to pilot the aircraft in a secure and professional manner despite the surrounding situation, and he must systematically apply the procedures in force.        According to the publication “OPERATIONS FOR AIRCRAFT AT ALTITUDES ABOVE 25,000 FT—(FAA-AC61-107A)”, the Effective Performance Time or EPT or the Time of Useful Consciousness or TUC is the time span during which a pilot is able to fulfil his duty in an effective or appropriate manner despite the rarefaction in oxygen. The value of the EPT decreases as a function of altitude until it coincides with the time necessary for the blood to pass from the lungs to the head and usually determined for an altitude of greater than 10,000 metres. The TUC is, for a standard ascent speed, from about 2.5 to 3 min (for an altitude of about 7,500 m) and from about 9 to 12 s (for an altitude of about 15,000 m), while it is, in the event of fast decompression, from 1.5 to 2.5 min and 5 s for the same altitudes, respectively.        
Three cases of aircraft that encountered meteorological problems or a probable depressurization problem and that crashed in mountains or in proximity to a town may be cited, for the year 2005, to illustrate the dangerousness of such emergency situations.
Among the existing onboard systems to which it would be possible to resort to aid the pilot in an emergency situation of the type of those cited above, may be cited:                The ISS/TAWS systems (“Terrain Awareness and Warning System” in the guise of autonomous computer or one integrated with the TCAS and WXR functions in an ISS “Integrated Surveillance System”) which fulfil a primary terrain anticollision monitoring function (“Safety Net”) and the aim of which is to emit audible alerts during an exceptional approach to the relief allowing the crew to react by engaging a vertical resource before it is too late. Accordingly, TAWS systems, decoupled from navigation systems, periodically compare the theoretical trajectory that the aircraft would describe during a resource and compare it with a section through the terrain overflown, obtained on the basis of a worldwide digital terrain model aboard the computer.        
The availability of a model of the terrain permits secondary functions making it possible to improve the perception of the situation of the crew (“Situation Awareness”). Among them, the THD (“Terrain Hazard Display”) is described in TSO-c151b of the TAWS and its objective is a representation of the vertical margins relating to the altitude of the aircraft as slices of false colours presented on the navigation screen. TAWSs of class A, compulsory for commercial transport aeroplanes, generally have a simplified cartographic mode with a few hypsometric slices (the ELEVIEW mode for the T2CAS from THALES), making it possible to have available a representation of the terrain during cruising flight phases.
The false colour representations are currently limited by the ARINC-453 display standards (of WXR type) and by the certification constraints which lead to an intentional degradation in the resolution of the graphical representations proposed so as not to allow their use for navigation, which is incompatible with the certification level defined for a TAWS.
The functions carried out by a TAWS are insufficient to make it possible, whatever the causes of an emergency situation (meteorology, depressurization, engine failure, navigation, medical emergency, etc.), to rapidly and definitively choose a valid descent trajectory until the aircraft is made safe so as to allow the crew to distribute its workload over the other tasks necessary for safeguards and for resolving the problems encountered.
Specifically, the THD proposes a terrain representation limited to 1500 feet under the aircraft, a margin which is largely insufficient to anticipate the dangers during a descent from a cruising regime. The known hypsometric representation devices propose a simplified cartographic representation dissociated from the concept of local safety altitude or values related to emergency descent procedures. These two display modes propose substantially horizontal relief sections for which the representation level is decorrelated from the ability of the aircraft to actually attain this zone, thereby rendering them unsuitable for representing the zones that the aircraft can reach in emergency situations such as those cited above.
Furthermore, the resolution of digital terrain models of the order of 15 seconds of arc (or less) is too high in regard to the operational margins required for the situations envisaged and in fact noncertifiable for navigation functions, and the data of the TAWS do not make it possible to ensure automatic tracking of the trajectory, or to access the navigation data, or to access the performance model to make predictions of (vertical profile, flight time and fuel consumption necessary for defining the zones and procedures that are achievable).
European patent application 0 826 946 discloses a method of assisting piloting, but this document pertains only to the avoidance of a dangerous zone and not to the formulation of the shortest possible safe emergency descent trajectory.